The neonatal Fc receptor (FcRn) is an intracellular trafficking integral membrane Fc receptor for IgG. FcRn was originally identified as a receptor functioning in neonatal life. It was first isolated from rodent gut as a heterodimer between a 12 kDa and a 40-50 kDa protein (Rodewald & Kraehenbuhl 1984, J. Cell. Biol. 99(1 Pt2): 159s-154s; Simister & Rees, 1985, Eur. J. Immunol. 15:733-738) and was cloned in 1989 (Simister & Mostov, 1989, Nature 337:184-187). Cloning and subsequent crystallization of FcRn revealed it to have an approximately 50 kDa major histocompatibility complex (MHC) class I-like heavy chain in non-covalent association with a 12 kDa β2-microglobulin light chain (Raghavan et al., 1993, Biochemistry 32:8654-8660; Huber et al., 1993, J. Mol. Biol. 230:1077-1083). Although first recognized in connection with fetal and neonatal life, FcRn is today known to continue to function throughout adult life. FcRn resides primarily in the early acidic endosomes where it binds to the Fc region of IgG in a pH-dependent manner, with micro- to nanomolar affinity at pH 6.5, while binding of FcRn to Fc at physiological pH is negligible. The bulk of FcRn is present in endosomes in most cells, and the interaction between FcRn and its IgG Fc ligands occurs within that acidic environment. In some cells, such as hematopoietic cells, significant levels of FcRn can be detected on the cell surface in addition to intracellular expression (Zhu et al., 2001, J. Immunol. 166:3266-3276). In this case, when the extracellular milieu is acidic, as in the case of neoplastic or infectious conditions, it is possible that FcRn can bind to IgG on the cell surface of these cell types. FcRn regulates serum IgG concentrations by binding to and protecting endocytosed monomeric IgG from degradation in the lysosomal compartment, and transporting the IgG to the cell surface for release at neutral extracellular pH. Through this mechanism, FcRn is responsible for the long serum half-life of IgG, since IgG that is not bound by FcRn enters the lysosomal pathway and is degraded.
During the first stages of life, FcRn confers passive immunity to offspring before and after birth by mediating transfer of IgG across the maternal placenta or neonatal intestinal walls. FcRn continues to function throughout adult life and is expressed in various tissues, e.g., the epithelium of the lung and liver, the vascular endothelium, as well as in monocytes, macrophages, and dendritic cells.
FcRn-deficient mice are more resistant to autoimmune diseases caused by pathogenic IgG autoantibodies because they are unable to maintain high concentrations of pathogenic serum IgG (Christianson et al., 1996, J. Immunol. 156:4932-4939; Ghetie et al., 1996, Eur. J. Immunol. 26:690-696; Israel et al., 1996, Immunol. 89:573-578). Administration of antibodies engineered to have modified Fc regions that bind with higher affinity to FcRn was found to ameliorate disease in a murine arthritis model (Patel et al., 2011, J. Immunol. 187:1015-1022). High dose administration of IgG in a number of autoimmune diseases has a palliative effect that can be explained at least partially by saturation of FcRn-mediated protection of IgG, shortening the half-life of pathogenic IgG (Jin & Balthasar, 2005, Hum. Immunol. 66:403-410; Akilesh et al., 2004, J. Clin. Invest. 113:1328-1333; Li et al., 2005, J. Clin. Invest. 115:3440-3450). Accordingly, specific blockade of FcRn-IgG interaction can be used to promote degradation of pathogenic IgG antibodies, for example to treat IgG mediated autoimmune diseases and to clear therapeutic antibodies from serum after administration. For example, in a rat model of experimentally-induced autoimmune myasthenia gravis, treatment with an FcRn heavy-chain specific monoclonal antibody resulted in a reduction of serum IgG concentration and a decrease in severity of the disease (Liu et al., 2007, J. Immunol. 178:5390-5398).
An absence of FcRn in hematopoeitic cells is associated with more rapid clearance of IgG containing immune complexes from the bloodstream (Qiao et al., 2008, Proc. Natl. Acad. Sci. USA 105: 9337-9342). This indicates that specific blockade of FcRn-IgG interactions will also promote the clearance of IgG containing immune complexes from the circulation.
FcRn regulates the movement of IgG, and any bound cargo, between different compartments of the body via transcytosis across polarized cells. This process plays an important role in mucosal protection from infection, e.g., in the gastrointestinal tract. FcRn transports IgG across the epithelial cell barrier of the intestines and into the lumen. After IgG binds antigen in the lumen, the IgG/antigen complex is transported back through the barrier by FcRn into the lamina propria, allowing for processing of the IgG/antigen complex by dendritic cells and presentation of antigen to CD4+ T cells in regional lymph nodes.
FcRn also plays a critical role in MHC class II antigen presentation and MHC class I cross-presentation of IgG-complexed antigen. When antigen is presented as an IgG-containing immune complex (IC), dendritic cells that are CD8-CD11b+CD11c+ (inflammatory dendritic cells) display significant cross-presentation at low antigen doses in a pathway that is highly dependent upon FcRn expression. This pathway involves the internalization of the ICs by Fcγ receptors into an acidic endosome. Subsequent binding of the ICs by FcRn within antigen presenting cells (APCs) initiates specific mechanisms that result in trafficking of the antigen-bearing IC into compartments where antigen is processed into peptide epitopes compatible with loading onto MHC (Baker et al., 2011, Proc. Natl. Acad. Sci. USA 108:9927-9932; Christianson et al., 2012, mAbs vol. 4, page 208, Introduction). Thus, FcRn in DCs enhances MHC II antigen presentation and induces proliferation of antigen-specific CD4+ T-cells as well as exhibiting a fundamental role in antigen presentation to CD8+ T cells (cytotoxic T cells). This latter CD8+ T cell-pathway is called cross-presentation and involves the crossover of extracellular antigens into an MHC class I-dependent pathway.
Blockade of FcRn-Ig IC interaction inhibits antigen presentation of IC and subsequent T cell activation stimulated by immune-associated antigen presentation. Interactions with IgG IC in APCs such as DCs also promote secretion of inflammatory cytokines such as IL-12, IFNγ, and TNFα. Thus, blockade of FcRn-Ig IC interaction is useful to inhibit production of inflammatory cytokines by innate immune cells and antigen activated T cells.
FcRn contains a binding site for serum albumin that is distinct from its binding site for the Fc domain of IgG, due to ionic interactions between FcRn and IgG or albumin on opposite faces of the FcRn heavy chain (Chaudhury et al., 2006, Biochemistry 45:4983-4990). Like its binding to IgG, binding of FcRn to albumin is strongly pH-dependent, occurring at acidic pH (typically less than pH 6, and optimally at pH 5) but not at neutral pH. Similar to its role in protecting IgG from degradation, FcRn binding of albumin protects albumin from degradation and results in an extended serum half-life for albumin.